In recent years, for a display device such as a mobile device such as a cell phone or a PDA, a touch panel or a liquid crystal television, a cover glass (protective glass) has been used in many cases in order to protect the display and to improve the appearance. Further, to a cover glass for a flat screen television such as a liquid crystal television, surface treatment, for example, formation of a film having a function such as antireflection, impact failure prevention, electromagnetic wave shielding, near infrared ray shielding or color tone correction may be applied in some cases.
For such a display device, weight reduction and thickness reduction are required for differentiation by a flat design or for reduction of the load for transportation. Therefore, a cover glass to be used for protecting a display is also required to be thin. However, if the thickness of the cover glass is made to be thin, the strength is lowered, and there has been a problem such that the cover glass itself is broken by e.g. a shock due to falling or flying of an object in the case of an installed type or by dropping during the use in the case of a portable device, and the cover glass cannot accomplish the essential role to protect a display device.
In order to solve the above problem, it is conceivable to improve the strength of the cover glass, and as such a method, a method to form a compressive stress layer on a glass surface is commonly known.
The method to form a compressive stress layer on a glass surface, may typically be an air quenching tempering method (physical tempering method) wherein a surface of a glass plate heated to near the softening point is quenched by air cooling or the like, or a chemical tempering method wherein alkali metal ions having a small ion radius (typically Li ions or Na ions) at a glass plate surface are exchanged with alkali ions having a larger ion radius (typically K ions) by ion exchange at a temperature lower than the glass transition point.
As mentioned above, the thickness of the cover glass is required to be thin. However, if the air quenching tempering method is applied to a thin glass plate having a thickness of less than 2 mm, as required for a cover glass, the temperature difference between the surface and the inside tends not to arise, and it is thereby difficult to form a compressive stress layer, and the desired property of high strength cannot be obtained. Therefore, a cover glass tempered by the latter chemical tempering method is usually used.
As such a cover glass, one having soda lime glass chemically tempered is widely used (e.g. Patent Document 1).
Soda lime glass is inexpensive and has a feature that the surface compressive stress S of a compressive stress layer formed at the surface of the glass by the chemical tempering can be made to be at least 550 MPa, but there has been a problem that it is difficult to make the thickness t of the compressive stress layer (hereinafter sometimes referred to as the compressive stress layer depth) to be at least 30 μm. Glass in the after-mentioned Example 27 is soda lime glass.
Therefore, one having SiO2—Al2O3—Na2O type glass different from soda lime glass, chemically tempered, has been proposed for such a cover glass (e.g. Patent Documents 2 and 3).
Such SiO2—Al2O3—Na2O type glass (hereinafter referred to as conventional glass) has a feature that it is possible not only to make the above S to be at least 550 MPa but also to make the above t to be at least 30 μm. Glasses in the after-mentioned Examples 28 and 29 are conventional glasses.